I see BGG largely as an encyclopaedic resource to find out about games. So my main criterion for adding an entry would be "are people in general possibly going to want to find out more about this game?" - if the answer is yes, then it should probably have an entry.

To my mind, to fit this criteria a game should be a) a complete, coherent game (there should be a game to find out about!), and b) not going to change significantly in the near future (so they find out actual information, not hype and speculation).

This probably means "after playtesting is more or less complete and the design is locked to any major changes", really.

For Print and Play games, I would add c) the game should be available to players in its final form, one way or another - because by "people in general" I mean "people who aren't watching your work in progress thread"; people who heard mention of it from a friend, people who saw it in a mail-order catalogue, people who are looking for another game of type X to buy and are browsing the categories... that kind of people in general.